doc/user/event-filtering.md: event filtering with Predicates

[estroz]

Description of the change: document predicate usage in SDK controllers.

Motivation for the change: improves documentation of controller features.

[hasbro17]

Great work. The doc looks good overall. Can we just extend the example a bit more to make it more illustrative:

pred := predicate.Funcs{
	UpdateFunc: func(e event.UpdateEvent) bool {
		// Ignore updates to CR status in which case metadata.Generation does not change
		return e.MetaOld.GetGeneration() != e.MetaNew.GetGeneration()
	},
	DeleteFunc: func(e event.DeleteEvent) bool {
		// Evaluates to false if the object has been confirmed deleted.
		return !e.DeleteStateUnknown
	},
}

[hasbro17]

LGTM